Although major depressive disorder (MDD) is a mood disorder, research examining the emotional experiences of individuals diagnosed with MDD has focused largely on the valence of mood, that is, on the extent to which individuals emotional experiences are positive or negative. In fact, emotional experiences can be parsed along several other dimensions such as variability, intensity, frequency, and duration. There is growing recognition that these dimensions are important for understanding the etiology and course of MDD. In this context, the proposed research will examine the emotional experiences of currently depressed and never-depressed adults, with a specific emphasis on affective instability, which encompasses emotional intensity, variability, frequency of change, and temporal dependency. The overarching goal of the proposed research is to utilize a diverse set of methods to test an affective model of MDD that integrates cognitive, interpersonal, and psychobiological factors. Participants everyday emotional experiences will be assessed using ecological momentary assessment (EMA) on handheld electronic devices that are programmed to randomly prompt them to complete a series of questions about their current emotional states. This method permits the examination of emotional experiences of depressed individuals in a naturalistic setting. Other aims include (1) examining two theoretically-derived factors that may underlie the proposed relation between affective instability and MDD (cognitive biases and unmet psychological need for relatedness); and (2) determining whether these emotional, interpersonal, and cognitive factors contribute to increased levels of systemic inflammation, that have been found to be associated with both depression and the progression of infectious, metabolic, and coronary diseases. To achieve these two objectives, participants cognitive biases (i.e., the tendency to recall negative (over positive) memories and to attend selectively to negative (over positive) stimuli) will be assessed using standardized computer tasks in a laboratory setting. In addition, as with affective instability, the extent to which participants psychological needs for relatedness are met will be assessed at each prompt during their week of EMA. Measuring both emotions and psychological needs over seven days will permit an examination of the coupling of these two constructs. Finally, using a relatively non-invasive technique, participants levels of systemic inflammation will be assessed twice, six months apart. These two assessments of inflammation will permit an examination of whether and how changes in inflammation are associated with affective, cognitive, and interpersonal factors measured at the first assessment. This study will help to elucidate the mechanisms that may underlie the relation between MDD and affective instability, and to gain a better understanding of how factors that increase individuals vulnerability to depression are associated with changes in systemic inflammation.